Case study of an active landslide at the flank of a water reservoir and its response during earthquakes

Abstract

Slow-moving active landslides are a widespread phenomenon and often cause major damage to infrastructure. Normally, such slow slides do not cause fatalities, but this can change during extreme events such as a heavy rain or a strong earthquake. In particular their response to earthquakes is only partially understood, while documented field observation of such cases are rare. Considering their mobile pre-seismic state, one would expect active slides to be highly susceptible to an acceleration during earthquakes. When such a landslide affects a flank of a water reservoir, the consequences of its collapse can be devastating due to subsequent tsunami waves overtopping the dam. In this article, a case study of a deep-seated slow-moving landslide with a volume of about 5 million m3 at the flank of a water reservoir in the Swiss Alps is presented. Extensive field investigations and monitoring allow a profound understanding of the kinematics of the landslides and its dependency on the hydrological conditions. Together with laboratory ring shear tests, this forms the basis for quantifying rate effects in the shear zone over a very wide velocity range and allows formulating and calibrating an accurate mechanical landslide model. The material point method is used to investigate potential landslide scenarios during different earthquakes. The simulations show rather small co-seismic displacements and even for pessimistic scenarios, a threat from tsunami waves seems unlikely. This implies that slow-moving active landslides can be less susceptible to co-seismic acceleration than stable slopes due to their ductile behaviour.